To get in the ballpark, bearing clearance and type as well as oil pump (std, high volume). Also planned use. To fine tune, I use hot oil pressure. I prefer to see at least 20 psi at idle and a steady regulated pressure over 2000 RPM.

On my stuff, I broke in with 10W30. Hot oil pressure was lower than I liked so I switched to 15W-40 and all is well.

With performance bearing clearances and full groove main bearings mine likes a little heavier oil. Also, I street drive mine and it can see extended hot weather idling in traffic from time to time... I didn't want to have to worry about the cam/lifters and top end getting enough oil in that situation. If it were race only I probably would have stayed with 10W30 for the slight performance gain.